U.S. patent application number 16/136462 was filed with the patent office on 2019-03-28 for vehicle control apparatus.
This patent application is currently assigned to JTEKT CORPORATION. The applicant listed for this patent is JTEKT CORPORATION. Invention is credited to Takashi KODERA.
Application Number | 20190092383 16/136462 |
Document ID | / |
Family ID | 63685665 |
Filed Date | 2019-03-28 |
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United States Patent
Application |
20190092383 |
Kind Code |
A1 |
KODERA; Takashi |
March 28, 2019 |
VEHICLE CONTROL APPARATUS
Abstract
A control apparatus calculates an axial force deviation, which
is a difference between an ideal axial force and an estimated axial
force. The ideal axial force is based on a target pinion angle of a
pinion shaft configured to rotate in association with a turning
operation of steered wheels. The estimated axial force is based on
a state variable (such as a current value of a steering operation
motor) that reflects vehicle behavior or a road condition. The
control apparatus changes a command value for a reaction motor in
response to the axial force deviation. For example, the control
apparatus includes a basic control circuit configured to calculate
a basic control amount, which is a fundamental component of the
command value. The basic control circuit changes the basic control
amount in response to the axial force deviation. The command value
based on the basic control amount reflects the road condition.
Inventors: |
KODERA; Takashi;
(Okazaki-shi, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
JTEKT CORPORATION |
Osaka |
|
JP |
|
|
Assignee: |
JTEKT CORPORATION
Osaka
JP
|
Family ID: |
63685665 |
Appl. No.: |
16/136462 |
Filed: |
September 20, 2018 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
B62D 5/0463 20130101;
B62D 6/008 20130101; B62D 6/10 20130101 |
International
Class: |
B62D 5/04 20060101
B62D005/04; B62D 6/00 20060101 B62D006/00; B62D 6/10 20060101
B62D006/10 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 27, 2017 |
JP |
2017-186606 |
Claims
1. A vehicle control apparatus configured to control a motor based
on a command value to be calculated based on a steering condition,
the motor being a source of a driving force to be applied to a
steering mechanism of a vehicle, the vehicle control apparatus
comprising: a first calculation circuit configured to calculate a
first component of the command value based on at least a steering
torque; a second calculation circuit configured to calculate a
target rotation angle of a rotating body based on a basic drive
torque, which is a total sum of the steering torque and the first
component, the rotating body being configured to rotate in
association with a turning operation of a steered wheel; and a
third calculation circuit configured to calculate a second
component of the command value through feedback control performed
so that an actual rotation angle of the rotating body equals the
target rotation angle, wherein the second calculation circuit
includes: an ideal axial force calculation circuit configured to
calculate an ideal axial force based on the target rotation angle;
an estimated axial force calculation circuit configured to
calculate, as an estimated axial force, an axial force applied to
the steered wheel based on a state variable that reflects vehicle
behavior or a road condition; a blending calculation circuit
configured to calculate a final axial force to be reflected in the
basic drive torque as a reaction force component for the basic
drive torque by summing up a value obtained by multiplying the
ideal axial force by a blending ratio and a value obtained by
multiplying the estimated axial force by a blending ratio, the
blending ratios being set individually based on the state variable
that reflects the vehicle behavior or the road condition or based
on the steering condition; and a subtractor configured to calculate
an axial force deviation, which is a difference between the ideal
axial force and the estimated axial force, and the first
calculation circuit is configured to change the first component of
the command value in response to the axial force deviation.
2. The vehicle control apparatus according to claim 1, wherein the
first calculation circuit includes: a basic control amount
calculation circuit configured to calculate a basic control amount
based on the steering condition, the basic control amount being a
fundamental component of the first component of the command value;
a compensation amount calculation circuit configured to calculate a
compensation amount for the fundamental component based on the
steering condition; and a calculator configured to calculate the
first component of the command value by summing up the fundamental
component and the compensation amount, and the first component of
the command value is changed by the basic control amount
calculation circuit changing the basic control amount in response
to the axial force deviation or the compensation amount calculation
circuit changing the compensation amount in response to the axial
force deviation.
3. The vehicle control apparatus according to claim 2, wherein it
is assumed that the compensation amount calculation circuit changes
the compensation amount in response to the axial three deviation,
the compensation amount calculation circuit includes a
stabilization control amount calculation circuit configured to
calculate, based on the steering condition, a stabilization control
amount for stabilizing a system, and the first component of the
command value is changed by the stabilization control amount
calculation circuit changing the stabilization control amount in
response to the axial force deviation.
4. The vehicle control apparatus according to claim 2, wherein it
is assumed that the compensation amount calculation circuit changes
the compensation amount in response to the axial force deviation,
the compensation amount calculation circuit includes a hysteresis
control amount calculation circuit configured to calculate, based
on the steering condition, a hysteresis control amount for
compensating a hysteresis characteristic caused by friction during
a steering operation, and the first component of the command value
is changed by the hysteresis control amount calculation circuit
changing the hysteresis control amount in response to the axial
force deviation.
5. The vehicle control apparatus according to claim 2, wherein it
is assumed that the compensation amount calculation circuit changes
the compensation amount in response to the axial three deviation,
the compensation amount calculation circuit includes a steering
wheel return control amount calculation circuit configured to
calculate, based on the steering condition, a steering wheel return
control amount for compensating a steering wheel return
characteristic, and the first component of the command value is
changed by the steering wheel return control amount calculation
circuit changing the steering wheel return control amount in
response to the axial force deviation.
6. The vehicle control apparatus according to claim 2, wherein it
is assumed that the compensation amount calculation circuit changes
the compensation amount in response to the axial force deviation,
the compensation amount calculation circuit includes a damping
control amount calculation circuit configured to calculate, based
on the steering condition, a damping control amount for
compensating a viscosity of the steering mechanism, and the first
component of the command value is changed by the damping control
amount calculation circuit changing the damping control amount in
response to the axial force deviation.
7. The vehicle control apparatus according to claim 1, wherein the
steering mechanism includes a steering operation shaft configured
to turn the steered wheel by applying a steering operation force
generated by a steering operation motor, and the estimated axial
three is one of the following estimated axial forces: a. a first
estimated axial force calculated based on a current value of the
steering operation motor; b. a second estimated axial three
calculated based on a lateral acceleration applied to the vehicle;
c. a third estimated axial force calculated based on a. yaw rate,
which is a speed at which the vehicle makes a turn; d. a fourth
estimated axial force obtained by summing up a value obtained by
multiplying the second estimated axial force by a blending ratio
and a value obtained by multiplying the third estimated axial force
by a blending ratio, the blending ratios being set individually
based on the vehicle behavior; and e. a fifth estimated axial force
obtained by summing up a value obtained by multiplying the first
estimated axial force by a blending ratio, a value obtained by
multiplying the second estimated axial force by a blending ratio,
and a value obtained by multiplying the third estimated axial force
by a blending ratio, the blending ratios being set individually
based on the vehicle behavior.
8. The vehicle control apparatus according to claim 1, wherein the
steering mechanism includes: a pinion shaft that serves as the
rotatii g body and is mechanically separated from a steering wheel;
and a steering operation shaft configured to turn the steered wheel
in association with rotation of the pinion shaft, and the steering
mechanism includes, as control targets: a reaction motor that
serves as the motor and is configured to generate, based on the
command value, a steering reaction force as the driving force to be
applied to the steering wheel, the steering reaction force being a
torque in a direction opposite to a steering direction; and a
steering operation motor configured to generate a steering
operation force for turning the steered wheel, the steering
operation force being applied to the pinion shaft or the steering
operation shaft.
9. The vehicle control apparatus according to claim 1, wherein the
steering mechanism includes: a pinion shaft that serves as the
rotating body and is configured to operate in association with a
steering wheel; and a steering operation shaft configured to turn
the steered wheel in association with rotation of the pinion shaft,
and the motor is an assist motor configured to generate a steering
assist three as the driving force to be applied to the steering
wheel, the steering assist force being a torque in a direction
identical to a steering direction.
Description
INCORPORATION BY REFERENCE
[0001] The disclosure of Japanese Patent Application No.
2017-186606 filed on Sep. 27, 2017 including the specification,
drawings and abstract, is incorporated herein by reference in its
entirety.
BACKGROUND OF THE INVENTION
1:. Field of the Invention
[0002] The present invention relates to a vehicle control
apparatus.
2. Description of the Related Art
[0003] Hitherto, there is known a so-called steer-by-wire type
steering system in which a steering wheel and steered wheels are
mechanically separated from each other. This steering system
includes a reaction motor and a steering operation motor. The
reaction motor is a source of a steering reaction force to be
applied to a steering shaft. The steering operation motor is a
source of a steering operation force for turning the steered
wheels. When a vehicle is traveling, a control apparatus of the
steering system generates the steering reaction force through the
reaction motor, and turns the steered wheels through the steering
operation motor.
[0004] In the steer-by-wire type steering system, it is not likely
that a road reaction force applied to the steered wheels is
transmitted to the steering wheel because the steering wheel and
the steered wheels are mechanically separated from each other.
Thus, it is difficult for the driver to grasp a road condition as
the steering reaction force (tactile feedback) that may be felt by
the hands through the steering wheel.
[0005] For example, a control apparatus described in Japanese
Patent Application Publication No. 2014-148299 (JP 2014-148299 A)
calculates a feedforward axial force and a feedback axial force.
The feedforward axial force is an ideal rack axial force based on a
steering angle. The feedback axial force is an estimated axial
force based on state variables of the vehicle (lateral
acceleration, steering operation current, and yaw rate), The
feedback axial force is calculated based on a blended axial force
obtained by summing up, at predetermined blending ratios, axial
forces calculated individually for the state variables of the
vehicle. The control apparatus calculates a final axial force by
summing up the feedforward axial force and the feedback axial force
at predetermined blending ratios, and controls the reaction motor
based on the final axial force. The feedback axial force reflects a
road condition (road information), and therefore the steering
reaction force generated by the reaction motor also reflects the
road information. Thus, the driver can grasp the road information
as the steering reaction force.
[0006] The driver is informed of accurate road information through
the steering wheel as tactile feedback, and can therefore perform a
steering operation more quickly and more accurately. Further, a
feeling of security in driving is enhanced. Therefore, a further
improvement is desired for informing the driver of the road
condition more appropriately as the steering reaction force
(tactile feedback).
SUMMARY OF THE INVENTION
[0007] It is one object of the present invention to provide a
vehicle control apparatus capable of informing a driver of a road
condition more appropriately as a steering reaction force.
[0008] A vehicle control apparatus according to one aspect of the
present invention is a vehicle control apparatus configured to
control a motor based on a command value to be calculated based on
a steering condition. The motor is a source of a driving force to
be applied to a steering mechanism of a vehicle. The vehicle
control apparatus includes a first calculation circuit, a second
calculation circuit, and a third calculation circuit. The first
calculation circuit is configured to calculate a first component of
the command value based on at least a steering torque. The second
calculation circuit is configured to calculate a target rotation
angle of a rotating body based on a basic drive torque, which is a
total sum of the steering torque and the first component. The
rotating body is configured to rotate in association with a turning
operation of a steered wheel. The third calculation circuit is
configured to calculate a second component of the command value
through feedback control performed so that an actual rotation angle
of the rotating body equals the target rotation angle.
[0009] The second calculation circuit includes an ideal axial force
calculation circuit, an estimated axial force calculation circuit,
a blending calculation circuit, and a subtractor. The ideal axial
force calculation circuit is configured to calculate an ideal axial
force based on the target rotation angle. The estimated axial force
calculation circuit is configured to calculate, as an estimated
axial force, an axial force applied to the steered wheel based on a
state variable that reflects vehicle behavior or a road condition.
The blending calculation circuit is configured to calculate a final
axial force to be reflected in the basic drive torque as a reaction
force component for the basic drive torque by summing up a value
obtained by multiplying the ideal axial force by a blending ratio
and a value obtained by multiplying the estimated axial force by a
blending ratio. The blending ratios are set individually based on
the state variable that reflects the vehicle behavior or the road
condition or based on the steering condition. The subtractor is
configured to calculate an axial force deviation, which is a
difference between the ideal axial force and the estimated axial
force. The first calculation circuit is configured to change the
first component of the command value in response to the axial force
deviation.
[0010] The axial force deviation between the ideal axial force and
the estimated axial force reflects the road condition. For example,
when the vehicle is traveling along a low-friction road, the axial
force deviation is likely to occur between the ideal axial force
and the estimated axial force. The axial force deviation has a
larger value as a road grip decreases. According to the
configuration described above, the first component of the command
value for the motor is changed in response to the axial force
deviation, thereby calculating a command value that reflects the
road condition more appropriately. Therefore, the motor generates a
driving force that reflects the road condition more appropriately.
Thus, the driver can acquire a more appropriate steering reaction
force in response to the road condition as tactile feedback.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] The foregoing and further features and advantages of the
invention will become apparent from the following description of
example embodiments with reference to the accompanying drawings,
wherein like numerals are used to represent like elements and
wherein:
[0012] FIG. 1 is a configuration diagram of a steer-by-wire type
steering system on which a vehicle control apparatus according to a
first embodiment is mounted;
[0013] FIG. 2 is a control block diagram of an electronic control
apparatus according to the first embodiment;
[0014] FIG. 3 is a control block diagram of a target steering
reaction force calculation circuit according to the first
embodiment;
[0015] FIG. 4 is a control block diagram of a target steering angle
calculation circuit according to the first embodiment;
[0016] FIG. 5 is a control block diagram of a vehicle model
according to the first embodiment;
[0017] FIG. 6 is a control block diagram of an axial force blending
calculation circuit according to the first embodiment;
[0018] FIG. 7 is a control block diagram of a basic control circuit
according to the first embodiment;
[0019] FIG. 8 is a control block diagram of a basic control circuit
according to a second embodiment;
[0020] FIG. 9 is a graph illustrating a relationship between a
steering torque and a basic control amount according to the second
embodiment;
[0021] FIG. 10 is a control block diagram of a system stabilization
control circuit according to the second embodiment;
[0022] FIG. 11 is a control block diagram of a hysteresis control
circuit according to a third embodiment;
[0023] FIG. 12 is a control block diagram of an ideal axial force
calculation circuit according to the third embodiment;
[0024] FIG. 13 is a control block diagram of a steering wheel
return control circuit according to a fourth embodiment;
[0025] FIG. 14 is a control block diagram of a damping control
circuit according to a fifth embodiment;
[0026] FIG. 15 is a configuration diagram of a steering system
(electric power steering system) according to a sixth embodiment;
and
[0027] FIG. 16 is a control block diagram of an electronic control
apparatus according to the sixth embodiment.
DETAILED DESCRIPTION OF EMBODIMENTS
[0028] Description is given of a vehicle control apparatus
according to a first embodiment of the present invention, which is
applied to a steer-by-wire type steering system.
[0029] As illustrated in FIG. 1, a steering system 10 for a vehicle
includes a steering shaft 12 coupled to a steering wheel 11. A
pinion shaft 13 is provided at the end of the steering shaft 12
that is located opposite to the steering wheel 11. Pinion teeth 13a
of the pinion shaft 13 mesh with rack teeth 14a of a steering
operation shaft 14 extending in a direction that intersects the
pinion shaft 13. Right and left steered wheels 16 and 16 are
coupled to both ends of the steering operation shaft 14 via tie
rods 15 and 15, respectively. The steering shaft 12, the pinion
shaft 13, and the steering operation shaft 14 function as a power
transmission path between the steering wheel 11 and each of the
steered wheels 16 and 16. That is, the steering operation shaft 14
performs linear motion along with a rotational operation of the
steering wheel 11, thereby changing a steered angle .theta.t of
each of the steered wheels 16 and 16.
[0030] The steering system 10 includes a clutch 21. The clutch 21
is provided on the steering shaft 12. An electromagnetic clutch is
employed as the clutch 21. The electromagnetic clutch connects and
disconnects power through connection and disconnection of electric
power for an exciting coil. When the clutch 21 is disengaged, the
power transmission path between the steering wheel 11 and each of
the steered wheels 16 and 16 is disconnected mechanically. When the
clutch 21 is engaged, the power transmission path between the
steering wheel 11 and each of the steered wheels 16 and 16 is
connected mechanically.
[0031] The steering system 10 includes a reaction motor 31, a speed
reducing mechanism 32, a rotation angle sensor 33, and a torque
sensor 34 as a structure for generating a steering reaction force
(reaction unit). The steering reaction force is a force (torque) to
be applied in a direction opposite to a direction of a driver's
operation of the steering wheel 11. By applying the steering
reaction force to the steering wheel 11, the driver can acquire
appropriate tactile feedback.
[0032] The reaction motor 31 is a source of the steering reaction
force. For example, a three-phase (U, V, W) brushless motor is
employed as the reaction motor 31. The reaction motor 31 (to be
exact, its rotation shaft) is coupled to the steering shaft 12 via
the speed reducing mechanism 32. The speed reducing mechanism 32 is
provided at a part of the steering shaft 12 that is located on the
steering wheel 11 side with respect to the clutch 21. A torque of
the reaction motor 31 is applied to the steering shaft 12 as the
steering reaction force.
[0033] The rotation angle sensor 33 is provided on the reaction
motor 31. The rotation angle sensor 33 detects a rotation angle
.theta..sub.a of the reaction motor 31. The rotation angle
.theta..sub.a, of the reaction motor 31 is used for calculating a
steering angle .theta..sub.s. The reaction motor 31 and the
steering shaft 12 operate in association with each other via the
speed reducing mechanism 32. Therefore, the rotation angle
.theta..sub.a of the reaction motor 31 is correlated to the
steering angle .theta..sub.s , that is a rotation angle of the
steering shaft 12 and furthermore a rotation angle of the steering
wheel 11. Thus, the steering angle .theta..sub.s can be determined
based on the rotation angle .theta..sub.a of the reaction motor
31.
[0034] The torque sensor 34 detects a steering torque T.sub.h
applied to the steering shaft 12 through a rotational operation of
the steering wheel 11. The torque sensor 34 is provided at a part
of the steering shaft 12 that is located on the steering wheel 11
side with respect to the speed reducing mechanism 32.
[0035] The steering system 10 includes a steering operation motor
41, a speed reducing mechanism 42, and a rotation angle sensor 43
as a structure for generating a steering operation force (steering
operation unit) that is power for turning the steered wheels 16 and
16.
[0036] The steering operation motor 41 is a source of the steering
operation force. For example, a three-phase brushless motor is
employed as the steering operation motor 41. The steering operation
motor 41 (to be exact, its rotation shaft) is coupled to a pinion
shaft 44 via the speed reducing mechanism 42. Pinion teeth 44a of
the pinion shaft 44 mesh with rack teeth 14b of the steering
operation shaft 14. A torque of the steering operation motor 41 is
applied to the steering operation shaft 14 via the pinion shaft 44
as the steering operation force. In response to rotation of the
steering operation motor 41, the steering operation shaft 14 moves
along a vehicle width direction (lateral direction in FIG. 1). The
rotation angle sensor 43 is provided on the steering operation
motor 41. The rotation angle sensor 43 detects a rotation angle
.theta..sub.b of the steering operation motor 41.
[0037] The steering system 10 includes a control apparatus 50. The
control apparatus 50 controls the reaction motor 31, the steering
operation motor 41, and the clutch 21 based on detection results
from various sensors. As the sensors, a vehicle speed sensor 501 is
provided in addition to the rotation angle sensor 33, the torque
sensor 34, and the rotation angle sensor 43 described above. The
vehicle speed sensor 501 is provided on the vehicle to detect a
vehicle speed V that is a traveling speed of the vehicle.
[0038] The control apparatus 50 executes engagement/disengagement
control for switching engagement and disengagement of the clutch 21
based on whether a clutch engagement condition is satisfied.
Examples of the clutch engagement condition include a condition
that a power switch of the vehicle is OFF. When the clutch
engagement condition is not satisfied, the control apparatus 50
switches the clutch 21 from an engaged state to a disengaged state
by energizing the exciting coil of the clutch 21. When the clutch
engagement condition is satisfied, the control apparatus 50
switches the clutch 21 from the disengaged state to the engaged
state by stopping the energization of the exciting coil of the
clutch 21.
[0039] The control apparatus 50 executes reaction control for
generating a steering reaction force based on the steering torque
T.sub.h through drive control for the reaction motor 31. The
control apparatus 50 calculates a target steering reaction force
based on at least the steering torque T.sub.h out of the steering
torque T.sub.h and the vehicle speed V, and calculates a target
steering angle of the steering wheel 11 based on the calculated
target steering reaction force, the steering torque T.sub.h, and
the vehicle speed V. The control apparatus 50 calculates a steering
angle correction amount through feedback control of the steering
angle .theta..sub.s, which is executed so that the actual steering
angle .theta..sub.s follows the target steering angle, and
calculates a steering reaction force command value by adding the
calculated steering angle correction amount to the target steering
reaction force. The control apparatus 50 supplies, to the reaction
motor 31, a current necessary to generate a steering reaction force
based on the steering reaction force command value.
[0040] The control apparatus 50 executes steering operation control
for turning the steered wheels 16 and 16 based on a steering
condition through drive control for the steering operation motor
41. The control apparatus 50 calculates a pinion angle
.theta..sub.p that is an actual rotation angle of the pinion shaft
44 based on the rotation angle .theta..sub.b of the steering
operation motor 41 that is detected through the rotation angle
sensor 43. The pinion angle .theta..sub.p is a value that reflects
the steered angle .theta..sub.t of each of the steered wheels 16
and 16. The control apparatus 50 calculates a target pinion angle
by using the target steering angle described above. Then, the
control apparatus 50 determines a deviation between the target
pinion angle and the actual pinion angle .theta..sub.p, and
controls power supply to the steering operation motor 41 so as to
eliminate the deviation.
[0041] Next, a detailed configuration of the control apparatus 50
is described. As illustrated in FIG. 2, the control apparatus 50
includes a reaction control circuit 50a configured to execute the
reaction control, and a steering operation control circuit 50b
configured to execute the steering operation control. The reaction
control circuit 50a includes a target steering reaction force
calculation circuit 51, a target steering angle calculation circuit
52, a steering angle calculation circuit 53, a steering angle
feedback control circuit 54, an adder 55, and an energization
control circuit 56.
[0042] The target steering reaction force calculation circuit 51
calculates a target steering reaction force T.sub.i* based on the
steering torque T.sub.h. The target steering reaction force
calculation circuit 51 may calculate the target steering reaction
force T.sub.1* in consideration of the vehicle speed V.
[0043] The target steering angle calculation circuit 52 calculates
a target steering angle .theta.* of the steering wheel 11 based on
the target steering reaction force T.sub.1*, the steering torque
T.sub.h, and the vehicle speed V. The target steering angle
calculation circuit 52 has an ideal model that defines an ideal
steering angle based on a basic drive torque (input torque), which
is the total sum of the target steering reaction force T.sub.1 and
the steering torque T.sub.h. The ideal model is obtained by
modeling a steering angle corresponding to an ideal steered angle
based on the basic drive torque through an experiment or the like
in advance. The target steering angle calculation circuit 52
determines the basic drive torque by adding the target steering
reaction force T.sub.1 and the steering torque T.sub.h together,
and calculates the target steering angle .theta.* from the basic
drive torque based on the ideal model.
[0044] The steering angle calculation circuit 53 calculates the
actual steering angle 8, of the steering wheel 11 based on the
rotation angle .theta..sub.a of the reaction motor 31 that is
detected through the rotation angle sensor 33. The steering angle
feedback control circuit 54 calculates a steering angle correction
amount T.sub.2* through feedback control of the steering angle
.theta..sub.s so that the actual steering angle .theta..sub.s
follows the target steering angle .theta.*. The adder 55 calculates
a steering reaction force command value T* by adding the steering
angle correction amount T.sub.2* to the target steering reaction
force T.sub.i*.
[0045] The energization control circuit 56 supplies electric power
to the reaction motor 31 based on the steering reaction force
command value T*. Specifically, the energization control circuit 56
calculates a current command value for the reaction motor 31 based
on the steering reaction force command value T*. The energization
control circuit 56 detects an actual current value I.sub.a
generated in a power supply path to the reaction motor 31 through a
current sensor 57 provided in the power supply path. The current
value I.sub.a is a value of an actual current supplied to the
reaction motor 31. Then, the energization control circuit 56
determines a deviation between the current command value and the
actual current value I.sub.a, and controls power supply to the
reaction motor 31 so as to eliminate the deviation (feedback
control of the current value I.sub.a). Thus, the reaction motor 31
generates a torque based on the steering reaction force command
value T*. The driver can acquire appropriate tactile feedback in
response to a road reaction force.
[0046] As illustrated in FIG. 2, the steering operation control
circuit 50b includes a pinion angle calculation circuit 61, a
steering angle ratio change control circuit 62, a differentiation
steering control circuit 63, a pinion angle feedback control
circuit 64, and an energization control circuit 65.
[0047] The pinion angle calculation circuit 61 calculates the
pinion angle .theta..sub.p that is an actual rotation angle of the
pinion shaft 13 based on the rotation angle .theta..sub.b of the
steering operation motor 41 that is detected through the rotation
angle sensor 43. As described above, the steering operation motor
41 and the pinion shaft 13 operate in association with each other
via the speed reducing mechanism 42. Therefore, there is a
correlation between the rotation angle .theta..sub.b of the
steering operation motor 41 and the pinion angle .theta..sub.p. By
using the correlation, the pinion angle .theta..sub.p can be
determined from the rotation angle .theta..sub.b of the steering
operation motor 41. As described above, the pinion shaft 13 meshes
with the steering operation shaft 14. Therefore, there is also a
correlation between the pinion angle .theta..sub.p and the movement
amount of the steering operation shaft 14.
[0048] That is, the pinion angle .theta..sub.p is a value that
reflects the steered angle .theta.t of each of the steered wheels
16 and 16.
[0049] The steering angle ratio change control circuit 62 sets a
steering angle ratio, which is the ratio of the steered angle
.theta.t to the steering angle .theta..sub.s, based on a traveling
condition of the vehicle (for example, the vehicle speed V), and
calculates a target pinion angle based on the set steering angle
ratio. The steering angle ratio change control circuit 62
calculates a target pinion angle .theta..sub.p* so that the steered
angle .theta.t increases relative to the steering angle
.theta..sub.s as the vehicle speed V decreases or that the steered
angle .theta.t decreases relative to the steering angle
.theta..sub.s as the vehicle speed V increases. In order to achieve
the steering angle ratio to be set based on the traveling condition
of the vehicle, the steering angle ratio change control circuit 62
calculates a correction angle for the target steering angle
.theta.*, and adds the calculated correction angle to the target
steering angle .theta.*, thereby calculating the target pinion
angle .theta..sub.p* based on the steering angle ratio,
[0050] The differentiation steering control circuit 63 calculates a
change speed of the target pinion angle .theta..sub.p* (steered
speed) by differentiating the target pinion angle .theta..sub.p*.
The differentiation steering control circuit 63 calculates a
correction angle for the target pinion angle .theta..sub.p* by
multiplying the change speed of the target pinion angle
.theta..sub.p* by a gain. The differentiation steering control
circuit 63 calculates a final target pinion angle .theta..sub.p* by
adding the correction angle to the target pinion angle
.theta..sub.p*. A delay in the steering operation is adjusted by
advancing the phase of the target pinion angle .theta..sub.p*
calculated by the steering angle ratio change control circuit 62.
That is, a steering operation response is secured based on the
steered speed.
[0051] The pinion angle feedback control circuit 64 calculates a
pinion angle command value T.sub.p* through feedback control
(proportional-integral-derivative (PID) control) of the pinion
angle .theta..sub.p so that the actual pinion angle .theta..sub.p
follows the final target pinion angle .theta..sub.p* calculated by
the differentiation steering control circuit 63.
[0052] The energization control circuit 65 supplies electric power
to the steering operation motor 41 based on the pinion angle
command value T.sub.p*. Specifically, the energization control
circuit 65 calculates a current command value for the steering
operation motor 41 based on the pinion angle command value
T.sub.p*. The energization control circuit 65 detects an actual
current value I.sub.b generated in a power supply path to the
steering operation motor 41 through a current sensor 66 provided in
the power supply path. The current value I.sub.b is a value of an
actual current supplied to the steering operation motor 41. Then,
the energization control circuit 65 determines a deviation between
the current command value and the actual current value I.sub.b, and
controls power supply to the steering operation motor 41 so as to
eliminate the deviation (feedback control of the current value
I.sub.b). Thus, the steering operation motor 41 rotates by an angle
based on the pinion angle command value T.sub.p*.
[0053] Next, the target steering reaction force calculation circuit
51 is described in detail. As illustrated in FIG. 3, the target
steering reaction force calculation circuit 51 includes a basic
control circuit 81, a system stabilization control circuit 82, a
hysteresis control circuit 83, a steering wheel return control
circuit 84, a damping control circuit 85, a calculator 86, and two
differentiators 87 and 88.
[0054] The differentiator 87 calculates a steering torque
derivative dT.sub.h by differentiating the steering torque T.sub.h.
The differentiator 88 calculates a steering speed .omega..sub.s by
differentiating the steering angle .theta..sub.s. The basic control
circuit 81 calculates a basic control amount I.sub.1* based on the
steering torque T.sub.h and the vehicle speed V. The basic control
amount I.sub.1* is a fundamental component (current value) for
generating a target steering reaction force of an appropriate
degree based on the steering torque T.sub.h and the vehicle speed
V.
[0055] The system stabilization control circuit 82 calculates a
stabilization control amount I.sub.2* (current value) based on the
steering torque derivative dT.sub.h and the vehicle speed V. The
stabilization control amount I.sub.2* is a compensation amount for
stabilizing the system by suppressing a resonance characteristic.
An overall control system of the steering system 10 is stabilized
by correcting the basic control amount I.sub.1* with the
stabilization control amount I.sub.2*.
[0056] The hysteresis control circuit 83 calculates a hysteresis
control amount I.sub.3* that is a compensation amount for extending
a tuning range of a target steering characteristic or a steering
feel or for optimizing a hysteresis characteristic caused by
friction during the steering operation. The hysteresis control
circuit 83 calculates the hysteresis control amount I.sub.3* based
on the steering angle .theta..sub.s and the vehicle speed V. The
hysteresis control amount I.sub.3* has a hysteresis characteristic
for a change in the steering angle .theta..sub.s.
[0057] The steering wheel return control circuit 84 calculates a
steering wheel return control amount I.sub.4* (current value) based
on the steering torque T.sub.h, the vehicle speed V, the steering
angle .theta..sub.s, and the steering speed .omega..sub.s. The
steering wheel return control amount I.sub.4* is a compensation
amount for compensating a return characteristic of the steering
wheel 11. By correcting the basic control amount I.sub.1* with the
steering wheel return control amount I.sub.4* compensation is made
for excess or deficiency of a self-aligning torque due to a road
reaction force. This is because a torque in a direction in which
the steering wheel 11 is returned to its neutral position is
generated based on the steering wheel return control amount
I.sub.4*.
[0058] The damping control circuit 85 calculates a damping control
amount I.sub.5*. (current value) based on the steering speed
.omega..sub.s and the vehicle speed V. The damping control amount
I.sub.5* is a compensation amount for compensating the viscosity of
the steering system 10. For example, small vibrations to be
transmitted to the steering wheel 11 are reduced by correcting the
basic control amount I.sub.1* with the damping control amount
I.sub.5*.
[0059] The calculator 86 calculates the target steering reaction
force T.sub.1* (current value) by adding the stabilization control
amount I.sub.2* and the steering wheel return control amount
I.sub.4* to the basic control amount I.sub.1* and subtracting the
hysteresis control amount I.sub.3* and the damping control amount
I.sub.5* from the basic control amount I.sub.1* as correction
processing for the basic control amount I.sub.1*.
[0060] Next, the target steering angle calculation circuit 52 is
described in detail. As described above, the target steering angle
calculation circuit 52 calculates the target steering angle
.theta.* based on the ideal model from the basic drive torque,
which is the total sum of the target steering reaction force
T.sub.1* and the steering torque T.sub.h. The ideal model is a
model using the fact that a basic drive torque T.sub.in* that is a
torque applied to the steering shaft 12 is represented by
Expression (1) below.
T.sub.in*=J.theta.*''+C.theta.*'+K.theta.* (1)
[0061] In Expression (1), "J" represents a moment of inertia of
each of the steering wheel 11 and the steering shaft 12, "C"
represents a coefficient of viscosity (coefficient of friction)
corresponding to, for example, friction of the steering operation
shaft 14 against a housing, and "K" represents a spring modulus
assuming the steering wheel 11 and the steering shaft 12 as
springs.
[0062] As understood from Expression (1), the basic drive torque
T.sub.in* is obtained by adding together a value obtained by
multiplying a second-order time derivative .theta.*'' of the target
steering angle .theta.* by the moment of inertia J, a value
obtained by multiplying a first-order time derivative .theta.*' of
the target steering angle .theta.* by the coefficient of viscosity
C, and a value obtained by multiplying the target steering angle
.theta.* by the spring modulus K. The target steering angle
calculation circuit 52 calculates the target steering angle
.theta.* in accordance with the ideal model based on Expression
(1).
[0063] As illustrated in FIG. 4, the ideal model based on
Expression (1) is divided into a steering model 71 and a vehicle
model 72. The steering model 71 is tuned based on characteristics
of the components of the steering system 10, such as the steering
shaft 12 and the reaction motor 31. The steering model 71 includes
an adder 73, a subtractor 74, an inertia model 75, a first
integrator 76, a second integrator 77, and a viscosity model
78.
[0064] The adder 73 calculates the basic drive torque T.sub.in* by
adding the target steering reaction force T.sub.1 * and the
steering torque T.sub.h together. The subtractor 74 calculates a
final basic drive torque T.sub.in* by subtracting a viscosity
component T.sub.vi* and a spring component T.sub.sp* described
later from the basic drive torque T.sub.in* calculated by the adder
73.
[0065] The inertia model 75 functions as an inertia control
calculation circuit corresponding to the inertia term of Expression
(1). The inertia model 75 calculates a steering angle acceleration
.alpha.* by multiplying the final basic drive torque T.sub.in*
calculated by the subtractor 74 and the inverse of the moment of
inertia J together.
[0066] The first integrator 76 calculates a steering angle speed
.omega.* by integrating the steering angle acceleration .alpha.*
calculated by the inertia model 75. The second integrator 77
calculates the target steering angle .theta.* by integrating the
steering angle speed .omega.* calculated by the first integrator
76. The target steering angle .theta.* is an ideal rotation angle
of the steering wheel 11 (steering shaft 12) based on the steering
model 71.
[0067] The viscosity model 78 functions as a viscosity control
calculation circuit corresponding to the viscosity term of
Expression (1). The viscosity model 78 calculates the viscosity
component T.sub.vi* of the basic drive torque T.sub.in* by
multiplying the steering angle speed .omega.* calculated by the
first integrator 76 and the coefficient of viscosity C
together.
[0068] The vehicle model 72 is tuned based on characteristics of
the vehicle on which the steering system 10 is mounted. A
vehicle-side characteristic that influences the steering
characteristic is determined based on, for example, specifications
of a suspension and wheel alignment and a grip force (friction
force) of each of the steered wheels 16 and 16. The vehicle model
72 functions as a spring characteristic control calculation circuit
corresponding to the spring term of Expression (1). The vehicle
model 72 calculates the spring component T.sub.sp* (spring reaction
torque) of the basic drive torque T.sub.in* by multiplying the
target steering angle .theta.* calculated by the second integrator
77 and the spring modulus K together.
[0069] When the vehicle model 72 calculates the spring component
T.sub.sp*, the vehicle speed V and the current value I.sub.b of the
steering operation motor 41 that is detected through the current
sensor 66 are taken into consideration. The vehicle model 72
acquires a pinion angle speed .omega..sub.p. The pinion angle speed
.omega..sub.p is obtained by the pinion angle .theta..sub.p
calculated by the pinion angle calculation circuit 61 being
differentiated by a differentiator 79 provided in the control
apparatus 50. The pinion shaft 13 meshes with the steering
operation shaft 14. Therefore, there is a correlation between a
change speed of the pinion angle .theta..sub.p (pinion angle speed
.omega..sub.p) and a moving speed of the steering operation shaft
14 (steered speed). That is, the pinion angle speed .omega..sub.p
is a value that reflects the steered speed of each of the steered
wheels 16 and 16. The steered speed may be determined from the
pinion angle speed .omega..sub.p by using the correlation between
the pinion angle speed .omega..sub.p and the steered speed.
[0070] According to the target steering angle calculation circuit
52 having the configuration described above, the relationship
between the basic drive torque T.sub.in* and the target steering
angle .theta.* can be tuned directly and furthermore a desired
steering characteristic can be achieved by adjusting the moment of
inertia J and the coefficient of viscosity C of the steering model
71 and the spring modulus K of the vehicle model 72.
[0071] The target pinion angle .theta..sub.p* is calculated by
using the target steering angle .theta.* calculated from the basic
drive torque T.sub.in* based on the steering model 71 and the
vehicle model 72. Then, feedback control is performed so that the
actual pinion angle .theta..sub.p equals the target pinion angle
.theta..sub.p*. As described above, there is a correlation between
the pinion angle .theta..sub.p and the steered angle .theta.t of
each of the steered wheels 16 and 16. Therefore, the turning
operation of each of the steered wheels 16 and 16 based on the
basic drive torque T.sub.in* is also determined by the steering
model 71 and the vehicle model 72. That is, the steering feel of
the vehicle is determined by the steering model 71 and the vehicle
model 72. Thus, a desired steering feel can be achieved by
adjusting the steering model 71 and the vehicle model 72.
[0072] The steering reaction force (tactile feedback to be acquired
through a steering operation), which is a force (torque) to be
applied in a direction opposite to the driver's steering direction,
is only based on the target steering angle .theta.*. That is, the
steering reaction force does not change in response to a road
condition (for example, the possibility of a slip that may occur on
a road). Therefore, it is difficult for the driver to grasp the
road condition through the steering reaction force. In this
example, the vehicle model 72 has the following configuration from
the viewpoint of addressing such concerns.
[0073] As illustrated in FIG. 5, the vehicle model 72 includes an
imaginary rack end axial force calculation circuit 90, an ideal
axial force calculation circuit 91, an estimated axial force
calculation circuit 92, an estimated axial force calculation
circuit 93, an estimated axial force calculation circuit 94, and an
axial force blending calculation circuit 95.
[0074] When the operation position of the steering wheel 11 is
close to a limit position of a physical operation range, the
imaginary rack end axial force calculation circuit 90 calculates an
imaginary rack end axial force F.sub.end as a correction amount for
the basic drive torque T.sub.in* in order to imaginarily limit the
operation range of the steering wheel 11 to a range narrower than
an original maximum physical steering range. The imaginary rack end
axial force F.sub.end is calculated from the viewpoint of sharply
increasing a torque (steering reaction torque) to be generated in
the reaction motor 31 in a direction opposite to the steering
direction.
[0075] The limit position of the physical operation range of the
steering wheel 11 is also a position at which the steering
operation shaft 14 reaches the limit of its movable range. When the
steering operation shaft 14 reaches the limit of its movable range,
there occurs a so-called "end abutment", in which the end of the
steering operation shaft 14 (rack end) abuts against the housing.
Therefore, the movement range of the rack shaft is physically
limited. Thus, the operation range of the steering wheel is also
limited.
[0076] The imaginary rack end axial force calculation circuit 90
acquires the target steering angle .theta.* and the target pinion
angle .theta..sub.p* calculated by the steering angle ratio change
control circuit 62 (see FIG. 2). The imaginary rack end axial force
calculation circuit 90 calculates a target steered angle by
multiplying the target pinion angle .theta..sub.p* by a
predetermined conversion coefficient. The imaginary rack end axial
force calculation circuit 90 compares the target steered angle with
the target steering angle .theta.*, and uses, as an imaginary rack
end angle .theta..sub.end, one of the target steered angle and the
target steering angle .theta.* that is larger in the absolute
value.
[0077] When the imaginary rack end angle .theta..sub.end reaches an
end determination threshold, the imaginary rack end axial force
calculation circuit 90 calculates the imaginary rack end axial
force F.sub.end by using an imaginary rack end map stored in a
storage apparatus (not illustrated) of the control apparatus 50.
The end determination threshold is set based on a value in the
vicinity of the maximum physical steering range of the steering
wheel 11 or a value in the vicinity of the maximum movable range of
the steering operation shaft 14. The imaginary rack end axial force
F.sub.end is a correction amount for the basic drive torque
T.sub.in*, and is set to have the same sign as the sign (positive
or negative) of the imaginary rack end angle .theta..sub.end. After
the imaginary rack end angle .theta..sub.end reaches the end
determination threshold, the imaginary rack end axial force
F.sub.end is set to a larger value as the absolute value of the
imaginary rack end angle .theta..sub.end end increases.
[0078] The ideal axial force calculation circuit 91 calculates an
ideal axial force which is an ideal value of the axial force to be
applied to the steering operation shaft 14 through the steered
wheels 16 and 16. The ideal axial force calculation circuit 91
calculates the ideal axial force F.sub.i by using an ideal axial
three map stored in the storage apparatus (not illustrated) of the
control apparatus 50. The ideal axial force F.sub.i is set to have
a larger absolute value as the absolute value of the target steered
angle obtained by multiplying the target pinion angle
.theta..sub.p* the predetermined conversion coefficient increases
and as the vehicle speed V decreases. The ideal axial force may be
calculated based on the target steered angle alone without
considering the vehicle speed V.
[0079] The estimated axial force calculation circuit 92 calculates
an actual axial force F1 (road reaction force) to be applied to the
steering operation shaft 14 (steered wheels 16 and 16) based on the
current value I.sub.b of the steering operation motor 41. The
current value I.sub.b of the steering operation motor 41 changes in
response to the occurrence of a difference between the target
pinion angle .theta..sub.p* and the actual pinion angle
.theta..sub.p due to a situation in which a disturbance caused by a
road condition (road frictional resistance) affects the steered
wheels 16. That is, the current value I.sub.b of the steering
operation motor 41 reflects an actual road reaction force applied
to the steered wheels 16 and 16. Therefore, an axial force that
reflects an influence of the road condition can be calculated based
on the current value I.sub.b of the steering operation motor 41.
The axial force F1 is determined by multiplying the current value
l.sub.b of the steering operation motor 41 by a gain that is a
coefficient for converting a current value into an axial force
(reaction torque).
[0080] The estimated axial force calculation circuit 93 estimates
and calculates an axial force F2 to be applied to the steering
operation shaft 14 based on a lateral acceleration LA detected
through a lateral acceleration sensor 502 provided in the vehicle.
The axial force F2 is determined by multiplying the lateral
acceleration LA by a gain that is a coefficient based on the
vehicle speed V. The lateral acceleration LA reflects a road
condition such as a road frictional resistance. Therefore, the
axial force F2 calculated based on the lateral acceleration LA
reflects an actual road condition.
[0081] The estimated axial force calculation circuit 94 estimates
and calculates an axial force F3 to be applied to the steering
operation shaft 14 based on a yaw rate YR detected through a yaw
rate sensor 503 provided in the vehicle. The axial force F3 is
determined by multiplying together a yaw rate derivative that is a
value obtained by differentiating the yaw rate YR and a vehicle
speed gain that is a coefficient based on the vehicle speed V. The
vehicle speed gain is set to a larger value as the vehicle speed V
increases. The yaw rate YR reflects a road condition such as a road
frictional resistance. Therefore, the axial force F3 calculated
based on the yaw rate YR reflects an actual road condition.
[0082] The axial force F3 may be calculated as follows. That is,
the estimated axial force calculation circuit 94 determines the
axial force F3 by adding at least one of a correction axial force
based on the steered angle .theta.t, a correction axial force based
on the steered speed, and a correction axial force based on a
steered angle acceleration to a value obtained by multiplying the
yaw rate derivative by the vehicle speed gain. The steered angle
.theta.t is obtained by multiplying the pinion angle .theta..sub.p
by a predetermined conversion coefficient. The steered speed may be
obtained by differentiating the steered angle .theta.t or by
converting the pinion angle speed to. The steered angle
acceleration may be obtained by differentiating the steered speed
or by converting a pinion angle acceleration .alpha..sub.p.
[0083] The axial three blending calculation circuit 95 calculates a
final axial force F.sub.sp by summing up the imaginary rack end
axial force F.sub.end, the ideal axial force F.sub.i, the axial
force F1 the axial three F2, and the axial force F3 at
predetermined blending ratios based on various state variables that
reflect a traveling condition or a steering condition of the
vehicle. The final axial force F.sub.sp is used for calculating the
spring component T.sub.sp* for the basic drive torque T.sub.n*. The
vehicle model 72 calculates (converts) the spring component
T.sub.sp* for the basic drive torque T.sub.in* based on the axial
force F.sub.sp.
[0084] Next, the axial force blending calculation circuit 95 is
described in detail. As illustrated in FIG. 6, the axial force
blending calculation circuit 95 includes a first calculation
circuit 95a and a second calculation circuit 95b.
[0085] The first calculation circuit 95a calculates a more
appropriate estimated axial force F.sub.e by summing up, at
predetermined blending ratios, the axial forces F1, F2, and F3
estimated and calculated by the estimated axial force calculation
circuits 92, 93, and 94, respectively.
[0086] The first calculation circuit 95a acquires the axial forces
F1, F2, and F3, the yaw rate YR, and a lateral acceleration
difference .DELTA.LA. The lateral acceleration difference .DELTA.LA
is calculated by a difference calculation circuit 96 provided in
the vehicle model 72. The difference calculation circuit 96
calculates the lateral acceleration difference .DELTA.LA based on
Expression (2) below.
.DELTA.LA=YR.times.V-LA (2)
[0087] In Expression (2), "YR" represents a yaw rate detected
through the yaw rate sensor 503, "V" represents a vehicle speed
detected through the vehicle speed sensor 501, and "LA" represents
a lateral acceleration detected through the lateral acceleration
sensor 502.
[0088] The first calculation circuit 95a includes an absolute value
calculation circuit 97, blending ratio calculation circuits 98 and
99, multipliers 101, 103, and 105, adders 102 and 106, and a
subtractor 104. The absolute value calculation circuit 97
calculates an absolute value |.DELTA.LA| of the lateral
acceleration difference .DELTA.LA calculated by the difference
calculation circuit 96. The blending ratio calculation circuit 98
calculates a blending ratio D.sub.a based on the absolute value
|.DELTA.LA| of the lateral acceleration difference .DELTA.LA. The
blending ratio D.sub.a is set to a larger value as the absolute
value |.DELTA.LA| of the lateral acceleration difference .DELTA.LA
increases and as the vehicle speed V increases. The multiplier 101
calculates an axial force F.sub.a after blending by multiplying the
axial force F3 based on the yaw rate YR and the blending ratio
D.sub.a together. The adder 102 calculates an axial force F.sub.b
by adding the axial force F2 based on the lateral acceleration LA
and the axial force F.sub.a calculated by the multiplier 101
together.
[0089] The blending ratio calculation circuit 99 calculates a
blending ratio D.sub.b based on the yaw rate YR. The blending ratio
D.sub.b is set to a larger value as the yaw rate YR increases and
as the vehicle speed V increases. The multiplier 103 calculates an
axial force F.sub.c by multiplying the axial force F.sub.b
calculated by the adder 102 and the blending ratio D.sub.b
together.
[0090] The subtractor 104 calculates a blending ratio D.sub.c by
subtracting the blending ratio D.sub.b calculated by the blending
ratio calculation circuit 99 from "1", which is a fixed value
stored in the storage apparatus of the control apparatus 50. The
multiplier 105 calculates an axial force F.sub.d by multiplying the
axial force F1 based on the current value I.sub.b of the steering
operation motor 41 and the blending ratio D.sub.c together.
[0091] The adder 106 calculates the final estimated axial force
F.sub.e by adding the axial force F.sub.d calculated by the
multiplier 105 and the axial force F.sub.c calculated by the
multiplier 103 together. The second calculation circuit 95b
calculates the final axial force F.sub.sp, which is used for
calculating the spring component T.sub.sp* for the basic drive
torque T.sub.in*, by summing up the estimated axial force F.sub.e
calculated by the first calculation circuit 95a and the ideal axial
force F.sub.i calculated by the ideal axial force calculation
circuit 91 at predetermined blending ratios based on various state
variables that reflect a traveling condition or a steering
condition of the vehicle.
[0092] The second calculation circuit 95b includes subtractors 107
and 117, blending ratio calculation circuits 108 to 114,
multipliers 115, 116, and 118, and adders 119 and 120.
[0093] The subtractor 107 calculates an axial force deviation
.DELTA.F by subtracting the estimated axial force F.sub.e
calculated in a blended manner by the first calculation circuit 95a
(adder 106) from the ideal axial force F.sub.i based on the target
pinion angle .theta..sub.p*.
[0094] The blending ratio calculation circuit 108 calculates a
blending ratio Dcc based on the axial force deviation .DELTA.F. The
blending ratio Dcc is set to a larger value as the axial force
deviation .DELTA.F increases. The blending ratio calculation
circuit 109 calculates a blending ratio D.sub.d based on the
imaginary rack end angle .theta..sub.end. The blending ratio
calculation circuit 110 calculates a blending ratio D.sub.c based
on the pinion angle speed .omega..sub.p (may be converted into the
steered speed). The blending ratio calculation circuit 111
calculates a blending ratio D.sub.f based on the steering speed
.omega..sub.s obtained by differentiating the steering angle
.theta..sub.s. The blending ratio calculation circuit 112
calculates a blending ratio D.sub.g based on the pinion angle
.theta..sub.p. The blending ratio calculation circuit 113
calculates a blending ratio D.sub.h based on the steering angle
.theta..sub.s. The blending ratio calculation circuit 114
calculates a blending ratio D.sub.i based on the vehicle speed V.
The blending ratios D.sub.d, D.sub.e, D.sub.f, D.sub.g, D.sub.h,
and D.sub.i are set to smaller values as the state variables
(.theta..sub.end, .omega..sub.p, .omega..sub.s, .theta..sub.p,
.theta..sub.s, V) acquired by the respective blending ratio
calculation circuits (109 to 114) increase.
[0095] The multiplier 115 calculates a blending ratio D.sub.j of
the final estimated axial three F.sub.e calculated by the first
calculation circuit 95a by multiplying the blending ratios Dcc,
D.sub.d, D.sub.e, D.sub.f, D.sub.g, D.sub.hand D.sub.i together.
The multiplier 116 calculates an estimated axial force F.sub.g
after blending by multiplying the final estimated axial force
F.sub.e calculated by the first calculation circuit 95a and the
blending ratio D.sub.j based on the state variables together.
[0096] The subtractor 117 calculates a blending ratio D.sub.k of
the ideal axial force F.sub.i by subtracting the blending ratio
D.sub.j calculated by the multiplier 115 from "1", which is a fixed
value stored in the storage apparatus of the control apparatus 50.
The multiplier 118 calculates an ideal axial force F.sub.h after
blending by multiplying the ideal axial force F.sub.i calculated by
the ideal axial force calculation circuit 91 and the blending ratio
D.sub.k together.
[0097] The adder 119 calculates an axial force F.sub.pre by summing
up the ideal axial force F.sub.h after blending and the estimated
axial force F.sub.g after blending. The adder 120 calculates the
final axial force F.sub.sp, which is used for calculating the
spring component T.sub.sp* for the basic drive torque T.sub.in*, by
summing up the axial force F.sub.pre calculated by the adder 119
and the imaginary rack end axial force F.sub.end. When the
imaginary rack end axial force F.sub.end is not calculated, the
axial force F.sub.pre calculated by the adder 119 is used as the
final axial force F.sub.sp, which is used for calculating the
spring component T.sub.sp* for the basic drive torque
T.sub.in*.
[0098] According to this embodiment, the axial forces F1, F2, and
F3 estimated and calculated based on the plurality of types of
state variables that reflect vehicle behavior or a road condition
and the ideal axial force F.sub.i calculated based on the target
pinion angle .theta..sub.p* (target steered angle) are summed up at
the blending ratios set based on the plurality of types of state
variables that reflect the vehicle behavior, the steering
condition, or the road condition. Thus, the axial force F.sub.pre
(F.sub.sp) that reflects the road condition more finely is
calculated. When the axial force F.sub.pre is reflected in the
basic drive torque T.sub.in*, a finer steering reaction force in
response to the road condition is applied to the steering wheel
11.
[0099] Depending on product specifications or the like, there is a
demand to further improve the performance of transmission of road
information in a tire grip limit range when the vehicle is
traveling along a low-friction road or the like. In this example,
the basic control circuit 81 has the following configuration.
[0100] As illustrated in FIG. 7, the basic control circuit 81
acquires the steering torque T.sub.h, the vehicle speed V, and the
axial force deviation .DELTA.F. The axial force deviation .DELTA.F
is a difference between the ideal axial force F.sub.i and the
estimated axial force F.sub.e, which is calculated by the axial
force blending calculation circuit 95 (see FIG. 6). The basic
control circuit 81 includes two gain calculation circuits 121 and
122 and two multipliers 123 and 124.
[0101] The gain calculation circuit 121 calculates a gain G1 by
using a map that defines a relationship between the steering torque
T.sub.h and the gain G1 depending on the vehicle speed V. The gain
G1 is set to a larger value as the steering torque T.sub.h
increases. The gain calculation circuit 122 calculates a gain G2 by
using a map that defines a relationship between the axial force
deviation .DELTA.F and the gain G2. The gain G2 is set to a smaller
value as the axial force deviation .DELTA.F increases. The gain
calculation circuit 122 may calculate the gain G2 in consideration
of the steering torque T.sub.h. The multiplier 123 calculates a
gain G3 by multiplying the gain G1 and the gain G2 together. The
multiplier 124 calculates the basic control amount I.sub.1* by
multiplying the steering torque T.sub.h and the gain G3
together.
[0102] The gain G3 reflects the gain G2 based on the axial force
deviation .DELTA.F. Therefore, the basic control amount I.sub.1*
obtained by multiplying the steering torque T.sub.h and the gain G3
together and furthermore the target steering reaction force
T.sub.1* based on the basic control amount I.sub.1* also reflect
the axial force deviation .DELTA.F.
[0103] Next, actions and effects of the basic control circuit 81
are described. For example, when the vehicle is traveling along a
low-friction road such as a wet road or a snowy road, the axial
force deviation .DELTA.F is likely to occur between the ideal axial
force F.sub.i and the estimated axial force F.sub.e. The reason is
as follows. That is, the ideal axial force F.sub.i is calculated
based on the target pinion angle .theta..sub.p*. Therefore, the
ideal axial force F.sub.i is not likely to reflect the road
condition. The estimated axial force F.sub.e is calculated based on
various state variables. Therefore, the estimated axial force
F.sub.e is likely to reflect the road condition. Thus, the ideal
axial force F.sub.i only has a value based on the target pinion
angle .theta..sub.p* irrespective of the tire grip condition,
whereas the estimated axial force F.sub.e decreases as the road
grip decreases. Accordingly, the difference between the ideal axial
force F.sub.i and the estimated axial three F.sub.e increases as
the road grip decreases. For this reason, the axial force deviation
.DELTA.F reflects the road condition.
[0104] As in this example, the basic control amount I.sub.1* is
changed in response to the axial force deviation .DELTA.F between
the ideal axial force F.sub.i and the estimated axial force
F.sub.e, thereby calculating a target steering reaction force
T.sub.1* that reflects the road condition more appropriately. Thus,
a more appropriate steering reaction force in response to the road
grip is applied to the steering wheel 11. The driver feels the
steering reaction force applied to the steering wheel 11 as tactile
feedback, and can therefore grasp the road condition more
accurately.
[0105] For example, the gain G2 is set to a smaller value as the
axial force deviation .DELTA.F increases along with a decrease in
the road grip of the tire. Therefore, the basic control amount
I.sub.1* and furthermore the target steering reaction force
T.sub.1* have smaller values. That is, the steering reaction force
to be applied to the steering shaft 12 decreases by an amount
corresponding to the subtraction of the hysteresis control amount
I.sub.3. The steering torque T.sub.h necessary to operate the
steering wheel 11 decreases by an amount corresponding to the
decrease in the steering reaction force. The driver can grasp, as
tactile feedback, a situation in which the road grip of the tire
decreases.
[0106] Next, a vehicle control apparatus according to a second
embodiment of the present invention is described. Depending on
product specifications or the like, there is a demand to secure a
system stability in the steering system 10 in the tire grip limit
range when the vehicle is traveling along a low-friction road or
the like. In this example, the target steering reaction force
calculation circuit 51 has the following configuration.
[0107] As illustrated in FIG. 8, the basic control circuit 81
includes the gain calculation circuit 121, the multiplier 124, and
a gradient calculation circuit 125. The multiplier 124 calculates
the basic control amount by multiplying the steering torque T.sub.h
and the gain G1 together. The gradient calculation circuit 125
calculates a gradient R based on the basic control amount I.sub.1*
and the steering torque T.sub.h. The gradient R is the rate of a
change in the basic control amount I.sub.1* relative to the
steering torque T.sub.h.
[0108] As illustrated in a graph of FIG. 9, for example, when the
steering torque T.sub.h, changes from a steering torque T.sub.h1 to
a steering torque T.sub.h2 and the basic control amount
I.sub.1*changes from a basic control amount I.sub.1-1* to a basic
control amount I.sub.1-2*, the gradient R is determined based on
Expression (3) below.
R=(I.sub.1-2*-I.sub.1-1*)/(T.sub.h2-T.sub.h1) (.sup.3)
[0109] As illustrated in FIG. 10, the system stabilization control
circuit 82 acquires the steering torque. T.sub.h, the vehicle speed
V, the gradient R, and the axial force deviation .DELTA.F. The
axial force deviation .DELTA.F is a difference between the ideal
axial force F.sub.i and the estimated axial force F.sub.e, which is
calculated by the axial force blending calculation circuit 95 (see
FIG. 6). The system stabilization control circuit 82 includes a
differentiator 131, two gain calculation circuits 132 and 133, and
two multipliers 134 and 135.
[0110] The differentiator 131 calculates the steering torque
derivative dT.sub.h by differentiating the steering torque T.sub.h.
The gain calculation circuit 132 calculates a gain G4 by using a
map that defines a relationship between the gradient R and the gain
G4 depending on the vehicle speed V. The gain G4 is set to a
smaller value as the gradient R increases. The gain calculation
circuit 133 calculates a gain G5 by using a map that defines a
relationship between the axial force deviation .DELTA.F and the
gain G5 depending on the vehicle speed V. The gain G5 is set to a
smaller value as the axial force deviation .DELTA.F increases. The
multiplier 134 calculates a gain G6 by multiplying the gain G4 and
the gain G5 together. The multiplier 135 calculates the
stabilization control amount I.sub.2* by multiplying the steering
torque derivative dT.sub.h and the gain G6 together.
[0111] The gain G6 reflects the gain G5 based on the axial force
deviation .DELTA.F. Therefore, the stabilization control amount
I.sub.2* obtained by multiplying the steering torque derivative
dT.sub.h and the gain G6 together also reflects the axial force
deviation .DELTA.F.
[0112] According to this example, the stabilization control amount
I.sub.2* is changed (increased or reduced) in response to the axial
force deviation .DELTA.F between the ideal axial force F.sub.i and
the estimated axial force F.sub.e, thereby attaining a more
appropriate stabilization control amount I.sub.2* based on the road
condition. Thus, the system stability in the steering system 10 can
be secured more appropriately based on the road condition. The
system stability in the tire grip limit range is also secured when
the vehicle is traveling along a low-friction road or the like.
[0113] Next, a vehicle control apparatus according to a third
embodiment is described. Depending on product specifications or the
like, there is a demand to reduce a sense of friction in the
steering system 10 in the tire grip limit range when the vehicle is
traveling along a low-friction road or the like. In this example,
the target steering reaction force calculation circuit 51 has the
following configuration.
[0114] As illustrated in FIG. 11, the hysteresis control circuit 83
includes a control amount calculation circuit 141, a gain
calculation circuit 142, and a multiplier 143.
[0115] The control amount calculation circuit 141 calculates the
hysteresis control amount I.sub.3* by using a map that defines a
relationship between the steering angle .theta..sub.s and the
hysteresis control amount I.sub.3*. The hysteresis control amount
I.sub.3* has a hysteresis characteristic for the steering angle
.theta..sub.s. The hysteresis characteristic in the map is set from
the viewpoint of optimizing a hysteresis characteristic caused by
friction in a mechanical transmission system of the steering system
10. The control amount calculation circuit 141 may calculate the
hysteresis control amount I.sub.3* in consideration of the vehicle
speed V.
[0116] The gain calculation circuit 142 calculates a gain G7 by
using a map that defines a relationship between the axial force
deviation .DELTA.F and the gain G7 depending on the vehicle speed
V. The gain G7 is set to a larger value as the axial force
deviation .DELTA.F increases.
[0117] The multiplier 143 calculates a final hysteresis control
amount I.sub.3* by multiplying the hysteresis control amount
I.sub.3* calculated by the control amount calculation circuit 141
and the gain G7 together. The gain G7 reflects the axial force
deviation .DELTA.F, and therefore the final hysteresis control
amount I.sub.3* also reflects the axial force deviation
.DELTA.F.
[0118] According to this example, the hysteresis control amount
I.sub.3* is changed in response to the axial force deviation
.DELTA.F between the ideal axial force F.sub.i and the estimated
axial force F.sub.e, thereby attaining a more appropriate
hysteresis control amount I.sub.3* (final) based on the road
condition. Thus, a steering feel (sense of friction in this case)
based on the hysteresis control amount I.sub.3* that reflects the
road condition can be provided for the driver. The driver feels the
steering reaction force applied to the steering wheel 11 as tactile
feedback, and can therefore grasp the sense of friction based on
the road condition more accurately.
[0119] For example, the final hysteresis control amount I.sub.3*
has a larger value as the axial force deviation .DELTA.F increases
along with a decrease in the road grip of the tire. This is because
a gain G7 having a larger value is calculated as the axial force
deviation .DELTA.F increases. Therefore, the target steering
reaction force T.sub.1* decreases by an amount corresponding to the
hysteresis control amount I.sub.3* subtracted from the basic
control amount I.sub.1*. That is, the steering reaction force to be
applied to the steering shaft 12 decreases by an amount
corresponding to the subtraction of the hysteresis control amount
I.sub.3*. Thus, a steering feel (sense of friction in this case)
based on the hysteresis control amount I.sub.3* can be provided for
the driver. The driver can grasp a decrease in the sense of
friction as the steering feel. Furthermore, the driver can grasp,
as tactile feedback, a situation in which the road grip of the tire
decreases.
[0120] In this example, the ideal axial force calculation circuit
91 may have the following configuration. As illustrated in FIG. 12,
the ideal axial force calculation circuit 91 includes an absolute
value calculation circuit 144, an axial force calculation circuit
145, a sign calculation circuit 146, a hysteresis calculation
circuit 147, a multiplier 148, and an adder 149.
[0121] The absolute value calculation circuit 144 calculates an
absolute value |.theta..sub.p*| of the target pinion angle
.theta..sub.p*. The axial force calculation circuit 145 calculates
the ideal axial force F.sub.i by using a map that defines a
relationship between the absolute value |.theta..sub.p*| of the
target pinion angle .theta..sub.p* and the ideal axial force
F.sub.i depending on the vehicle speed V. The ideal axial force
F.sub.i is set to a larger value as the absolute value
|.theta..sub.p*| of the target pinion angle .theta..sub.p*
increases.
[0122] The sign calculation circuit 146 calculates a sign S based
on the target pinion angle .theta..sub.p*. Specifically, Conditions
(A1) to (A3) below are applied.
[0123] (A1) When ".theta..sub.p*>0", the sign S is "1".
[0124] (A2) When ".theta..sub.p*=0", the sign S is "0".
[0125] (A3) When ".theta..sub.p*<0", the sign S is "-1".
[0126] The hysteresis calculation circuit 147 has a calculation
function similar to that of the hysteresis control circuit 83
illustrated in FIG. 11. That is, the hysteresis calculation circuit
147 calculates the hysteresis control amount I.sub.3* (final) based
on the steering angle .theta..sub.s, the vehicle speed V, and the
axial force deviation .DELTA.F.
[0127] The multiplier 148 multiplies the ideal axial force F.sub.i
calculated by the axial force calculation circuit 145 and the sign
S calculated by the sign calculation circuit 146 together. The
adder 149 calculates a final ideal axial force F.sub.i by adding
the ideal axial force F.sub.i multiplied by the sign S and the
hysteresis control amount I.sub.3* calculated by the hysteresis
calculation circuit 147 together.
[0128] By adding the hysteresis control amount I.sub.3* based on
the axial force deviation .DELTA.F to the ideal axial three F.sub.i
multiplied by the sign S, the final ideal axial three F.sub.i is
even closer to the actual axial force (estimated axial force). The
final axial force F,.sub.sp calculated by the axial three blending
calculation circuit 95 reflects the estimated axial force with
higher superiority to the ideal axial force F.sub.i.
[0129] Next, a vehicle control apparatus according to a fourth
embodiment is described. Depending on product specifications or the
like, there is a demand to further improve the steering wheel
return performance in the tire grip limit range when the vehicle is
traveling along a low-friction road or the like in this example,
the target steering reaction force calculation circuit 51 has the
following configuration.
[0130] As illustrated in FIG. 13, the steering wheel return control
circuit 84 includes a target speed calculation circuit 151, four
gain calculation circuits 152, 156, 157, and 158, four multipliers
153, 159, 160, and 161, a subtractor 154, and a control amount
calculation circuit 155.
[0131] The target speed calculation circuit 151 calculates a target
speed .omega..sub.s* by using a map that defines a relationship
between the steering angle .theta..sub.s and the target speed
.omega..sub.s* depending on the vehicle speed V. The target speed
.omega..sub.s* is a target value of a speed when the steering wheel
11 is returned to its neutral position. The target speed
.omega..sub.s* is set to a larger value as the steering angle
.theta..sub.s (absolute value) increases.
[0132] The gain calculation circuit 152 calculates a gain G8 by
using a map that defines a relationship between the axial force
deviation .DELTA.F and the gain G8 depending on the vehicle speed
V. The gain G8 is set to a larger value as the axial three
deviation .DELTA.F increases.
[0133] The multiplier 153 calculates a final target speed
.omega..sub.s* by multiplying the target speed .omega..sub.s* and
the gain G8 together. The gain G8 reflects the axial force
deviation .DELTA.F, and therefore the final target speed to
.omega..sub.s* also reflects the axial force deviation
.DELTA.F.
[0134] The subtractor 154 calculates a speed deviation
.DELTA..omega..sub.s by subtracting the steering speed
.omega..sub.s from the final target speed co.sub.s*. The control
amount calculation circuit 155 calculates the steering wheel return
control amount I.sub.4* by using a map that defines a relationship
between the speed deviation .DELTA..omega..sub.s and the steering
wheel return control amount I.sub.4* depending on the vehicle speed
V. The steering wheel return control amount I.sub.4* is set to a
larger value as the speed deviation .DELTA..omega..sub.s
increases.
[0135] The gain calculation circuit 156 calculates a gain G9 by
using a map that defines a relationship between the axial force
deviation .DELTA.F and the gain G9 depending on the vehicle speed
V. The gain G9 is set to a larger value as the axial force
deviation .DELTA.F increases.
[0136] The gain calculation circuit 157 calculates a gain G10 by
using a map that defines a relationship between the steering torque
T.sub.h and the gain G10 depending on the vehicle speed V. The gain
G10 is set to a smaller value as the steering torque T.sub.h
increases.
[0137] The gain calculation circuit 158 calculates a gain G11 by
using a map that defines a relationship between the axial force
deviation .DELTA.F and the gain G11 depending on the vehicle speed
V. The gain G11 is set to a larger value as the axial force
deviation .DELTA.F increases.
[0138] The multiplier 159 multiplies the steering wheel return
control amount I.sub.4* and the gain G9 together. The gain G9
reflects the axial force deviation .DELTA.F, and therefore the
steering wheel return control amount I.sub.4* multiplied by the
gain G9 has a value based on the axial force deviation
.DELTA.F.
[0139] The multiplier 160 calculates a gain G12 by multiplying the
gain G10 and the gain G11 together. That is, the gain G10 based on
the steering torque T.sub.h is changed in response to the axial
force deviation .DELTA.F.
[0140] The multiplier 161 calculates a final steering wheel return
control amount I.sub.4* by multiplying the steering wheel return
control amount I.sub.4* multiplied by the gain G9 and the gain G12
together.
[0141] According to this embodiment, the steering wheel return
control amount I.sub.4 * is changed in response to the axial force
deviation .DELTA.F, thereby attaining a more appropriate steering
wheel return control amount I.sub.4* based on the road condition.
For example, a steering wheel return control amount I.sub.4* having
a larger value calculated as the axial force deviation .DELTA.F
increases due to a decrease in the road grip of the tire. Thus, the
steering wheel return performance in the tire grip limit range can
be improved.
[0142] The fourth embodiment may be modified as follows.
[0143] The steering wheel return control circuit 84 may have a
configuration provided only with the control amount calculation
circuit 155, the gain calculation circuit 156, and the multiplier
159. That is, the target speed calculation circuit 151, the gain
calculation circuits 152, 157, and 158, the multipliers 153, 160,
and 161, and the subtractor 154 are omitted from the steering wheel
return control circuit 84 illustrated in FIG. 13. The control
amount calculation circuit 155 calculates the steering wheel return
control amount I.sub.4* by using a map that defines a relationship
between the steering angle .theta..sub.s and the steering wheel
return control amount I.sub.4* depending on the vehicle speed V.
The multiplier 159 calculates the final steering wheel return
control amount I.sub.4* by multiplying the steering wheel return
control amount I.sub.4* calculated by the control amount
calculation circuit 155 and the gain G9 calculated by the gain
calculation circuit 156 together.
[0144] Next, a vehicle control apparatus according to a fifth
embodiment is described. Depending on product specifications or the
like, there is a demand to reduce a sense of viscosity in the
steering system 10 in the tire grip limit range when the vehicle is
traveling along a low-friction road or the like, in this example,
the target steering reaction force calculation circuit 51 has the
following configuration.
[0145] As illustrated in FIG. 14, the damping control circuit 85
includes a control amount calculation circuit 171, five gain
calculation circuits 172, 173, 174, 175, and 176, and four
multipliers 177, 178, 179 and 180.
[0146] The control amount calculation circuit 171 calculates the
damping control amount I.sub.5* by using a map that defines a
relationship between the steering speed .omega..sub.s and the
damping control amount I.sub.5* depending on the vehicle speed V.
The damping control amount I.sub.5* is set to a larger value as the
steering speed .omega..sub.s increases.
[0147] The gain calculation circuit 172 calculates a gain G13 by
using a map that defines a relationship between the axial force
deviation .DELTA.F and the gain G13 depending on the vehicle speed
V. The gain G13 is set to a larger value as the axial force
deviation .DELTA.F increases.
[0148] The gain calculation circuit 173 calculates a gain G14 by
using a map that defines a relationship between the steering angle
.theta..sub.s and the gain G14 depending on the vehicle speed V.
The gain G14 is set to a smaller value as the steering angle
.theta..sub.s (absolute value) increases.
[0149] The gain calculation circuit 174 calculates a gain G15 by
using a map that defines a relationship between the axial force
deviation .DELTA.F and the gain G15 depending on the vehicle speed
V. A gain G15 having a larger value is calculated as the axial
force deviation .DELTA.F increases,
[0150] The gain calculation circuit 175 calculates a gain G16 by
using a map that defines a relationship between the steering torque
T.sub.h and the gain G16 depending on the vehicle speed V. The gain
G16 is set to a smaller value as the steering torque T.sub.h
(absolute value) increases.
[0151] The gain calculation circuit 176 calculates a gain G17 by
using a map that defines a relationship between the axial force
deviation .DELTA.F and the gain G17 depending on the vehicle speed
V. The gain G17 is set to a larger value as the axial force
deviation .DELTA.F increases.
[0152] The multiplier 177 multiplies the damping control amount
I.sub.5* calculated by the control amount calculation circuit 171
and the gain G13 calculated by the gain calculation circuit 172
together. The multiplier 178 calculates a gain G18 by multiplying
the gain G14 and the gain G15 together. The multiplier 179
calculates a gain G19 by multiplying the gain G16 and the gain G17
together. The multiplier 180 calculates a final damping control
amount I.sub.5* by multiplying the damping control amount I.sub.5*
multiplied by the gain G13, the gain G18, and the gain G19
together.
[0153] According to this embodiment, the damping control amount
I.sub.5* is changed in response to the axial force deviation
.DELTA.F between the ideal axial force F.sub.i and the estimated
axial force F.sub.e, thereby attaining a more appropriate damping
control amount I.sub.5* (final) based on the road condition. Thus,
a more appropriate steering reaction force in response to the road
condition is applied to the steering wheel 11. The driver feels the
steering reaction force applied to the steering wheel 11 as tactile
feedback, and can therefore grasp the road condition more
accurately.
[0154] For example, the final damping control amount I.sub.5* has a
larger value as the axial force deviation .DELTA.F increases along
with a decrease in the road grip of the tire. This is because a
gain G13 having a larger value is calculated as the axial force
deviation .DELTA.F increases. Therefore, the target steering
reaction force T.sub.1* decreases by an amount corresponding to the
increase in the damping control amount I.sub.5* subtracted from the
basic control amount I.sub.1*. That is, the steering reaction force
to be applied to the steering shaft 12 decreases by an amount
corresponding to the increase in the damping control amount I.sub.5
*. Thus, a steering feel (sense of viscosity in this case) based on
the damping control amount I.sub.5* can be provided for the driver.
The driver can grasp a decrease in the sense of viscosity as the
steering feel. Furthermore, the driver can grasp, as tactile
feedback, a situation in which the road grip of the tire
decreases.
[0155] The damping control amount I.sub.5* is adjusted by being
multiplied by the gain G14 based on the steering angle
.theta..sub.s and the gain G16 based on the steering torque
T.sub.h. The gains G14 and G16 are also changed in response to the
axial force deviation .DELTA.F. Thus, a more appropriate damping
control amount I.sub.5* based on the road condition can be
attained.
[0156] Next, description is given of a vehicle control apparatus
according to a sixth embodiment, which is applied to an electric
power steering system (hereinafter abbreviated as "EPS"). Members
similar to those of the first embodiment are represented by the
same reference symbols to omit their detailed description.
[0157] As illustrated in FIG. 15, an EPS 190 includes the steering
shaft 12, the pinion shaft 13, and the steering operation shaft 14
that function as the power transmission path between the steering
wheel 11 and each of the steered wheels 16 and 16. Reciprocating
linear motion of the steering operation shaft 14 is transmitted to
the right and left steered wheels 16 and 16 via the tie rods 15
coupled to both ends of the steering operation shaft 14,
respectively.
[0158] The EPS 190 includes an assist motor 191, a speed reducing
mechanism 192, the torque sensor 34, a rotation angle sensor 193,
and a control apparatus 194 as a structure for generating a
steering assist force (assist force). The rotation angle sensor 193
is provided on the assist motor 191 to detect its rotation angle
.theta..sub.m.
[0159] The assist motor 191 is a source of the steering assist
force. For example, a three-phase brushless motor is employed as
the assist motor 191. The assist motor 191 is coupled to the pinion
shaft 13 via the speed reducing mechanism 192. The speed of
rotation of the assist motor 191 is reduced by the speed reducing
mechanism 192, and a rotational force obtained through the speed
reduction is transmitted to the pinion shaft 13 as the steering
assist force.
[0160] The control apparatus 194 executes assist control for
generating a steering assist force based on the steering torque
T.sub.h through energization control for the assist motor 191. The
control apparatus 194 controls power supply to the assist motor 191
based on the steering torque T.sub.h detected through the torque
sensor 34, the vehicle speed V detected through the vehicle speed
sensor 501, and the rotation angle .theta..sub.m detected through
the rotation angle sensor 193.
[0161] As illustrated in FIG. 16, the control apparatus 194
includes a pinion angle calculation circuit 201, a basic assist
component calculation circuit 202, a target pinion angle
calculation circuit 203, a pinion angle feedback control circuit
(pinion angle F/B control circuit) 204, an adder 205, and an
energization control circuit 206.
[0162] The pinion angle calculation circuit 201 acquires the
rotation angle .theta..sub.m of the assist motor 191, and
calculates a pinion angle .theta..sub.p that is the rotation angle
of the pinion shaft 13 based on the acquired rotation angle
.theta..sub.m.
[0163] The basic assist component calculation circuit 202 basically
has a configuration similar to that of the target steering reaction
force calculation circuit 51 illustrated in FIG. 3. That is, the
basic assist component calculation circuit 202 includes the basic
control circuit 81, the system stabilization control circuit 82,
the hysteresis control circuit 83, the steering wheel return
control circuit 84, the damping control circuit 85, and the
calculator 86. The calculator 86 calculates a basic assist
component T.sub.a1* (current value) by adding the stabilization
control amount I.sub.2* and the steering wheel return control
amount I.sub.4* to the basic control amount I.sub.1* and
subtracting the hysteresis control amount I.sub.3* and the damping
control amount I.sub.5* from the basic control amount
[0164] In this example, the increase/decrease characteristic of the
map that is used in the gain calculation circuit 122 (see FIG. 7)
of the basic control circuit 81 is set opposite to the
characteristic of the first embodiment. That is, the gain
calculation circuit 122 calculates a gain G2 having a larger value
as the axial force deviation .DELTA.F increases.
[0165] The target pinion angle calculation circuit 203 acquires the
basic assist component T.sub.a1* calculated by the basic assist
component calculation circuit 202 and the steering torque T.sub.h.
The target pinion angle calculation circuit 203 has an ideal model
that defines an ideal pinion angle based on a basic drive torque
(input torque), which is the total sum of the basic assist
component T.sub.a1* and the steering torque T.sub.h. The ideal
model is obtained by modeling a pinion angle corresponding to an
ideal steered angle based on the basic drive torque through an
experiment or the like in advance. The target pinion angle
calculation circuit 203 determines the basic drive torque by adding
the basic assist component T.sub.a1* and the steering torque
T.sub.h together, and calculates a target pinion angle
.theta..sub.p*, from the determined basic drive torque based on the
ideal model. When the target pinion angle calculation circuit 203
calculates the target pinion angle .theta..sub.p*, the vehicle
speed V and a current value I.sub.m are taken into consideration.
The current value I.sub.m is detected through a current sensor 207
provided in a power supply path to the assist motor 191. The
current value I.sub.m is a value of an actual current supplied to
the assist motor 191.
[0166] The pinion angle feedback control circuit 204 acquires the
target pinion angle .theta..sub.p* calculated by the target pinion
angle calculation circuit 203 and the actual pinion angle
.theta..sub.p calculated by the pinion angle calculation circuit
201. The pinion angle feedback control circuit 204 performs
proportional-integral-derivative (PID) control as feedback control
for the pinion angle so that the actual pinion angle .theta..sub.p
follows the target pinion angle .theta..sub.p*. That is, the pinion
angle feedback control circuit 204 determines a deviation between
the target pinion angle .theta..sub.p* and the actual pinion angle
.theta..sub.p, and calculates correction component T.sub.a2* for
the basic assist component T.sub.a1* so as to eliminate the
deviation.
[0167] The adder 205 calculates an assist command value T.sub.a* by
adding the correction component T.sub.a2* to the basic assist
component T.sub.a1*. The assist command value T.sub.a* is a command
value indicating a rotational force (assist torque) to be generated
in the assist motor 191.
[0168] The energization control circuit 206 supplies electric power
to the assist motor 191 based on the assist command value T.sub.a*.
Specifically, the energization control circuit 206 calculates a
current command value for the assist motor 191 based on the assist
command value T.sub.a*. The energization control circuit 206
acquires the current value I.sub.m detected through the current
sensor 207. Then, the energization control circuit 206 determines a
deviation between the current command value and the actual current
value I.sub.m, and controls power supply to the assist motor 191 so
as to eliminate the deviation. Thus, the assist motor 191 generates
a torque based on the assist command value T.sub.a*. As a result,
the steering is assisted based on the steering condition.
[0169] According to the EPS 190, the target pinion angle
.theta..sub.p* is set based on the ideal model from the basic drive
torque (total sum of the basic assist component T.sub.a1*: and the
steering torque T.sub.h), and feedback control is performed so that
the actual pinion angle .theta..sub.p equals the target pinion
angle .theta..sub.p*. As described above, there is a correlation
between the pinion angle .theta..sub.p and the steered angle
.theta.t of each of the steered wheels 16 and 16. Therefore, the
turning operation of each of the steered wheels 16 and 16 based on
the basic drive torque is also determined by the ideal model. That
is, the steering feel of the vehicle is determined by the ideal
model. Thus, a desired steering feel can be achieved by adjusting
the ideal model.
[0170] The actual steered angle .theta.t is kept as a steered angle
.theta.t based on the target pinion angle .theta..sub.p*.
Therefore, there is attained an effect of suppressing a reverse
input vibration that may be caused by a disturbance due to a road
condition, braking, or the like. That is, even if a vibration is
transmitted to a steering mechanism such as the steering shaft 12
via the steered wheels 16 and 16, the correction component
T.sub.a2* is adjusted so that the pinion angle .theta..sub.p equals
the target pinion angle .theta..sub.p*. Therefore, the actual
steered angle .theta.t is kept as a steered angle et based on the
target pinion angle .theta..sub.p*; defined by the ideal model.
Consequently, the transmission of the reverse input vibration to
the steering wheel 11 is suppressed by assisting the steering in a
direction in which the reverse input vibration is canceled.
[0171] The steering reaction force (tactile feedback to be acquired
through a steering operation), which is a force (torque) to be
applied in a direction opposite to the driver's steering direction,
is only based on the target pinion angle .theta..sub.p*. That is,
the steering reaction force does not change in response to a road
condition such as a dry road or a low-friction road. Therefore, it
is difficult for the driver to grasp the road condition as tactile
feedback.
[0172] In this example, the calculation function of the target
steering angle calculation circuit 52 of the first embodiment is
provided to, for example, the target pinion angle calculation
circuit 203.
[0173] The target pinion angle calculation circuit 203 has a
functional configuration similar to that of the target steering
angle calculation circuit 52 illustrated in FIG. 4. The target
steering angle calculation circuit 52 acquires the target steering
reaction force T.sub.1*, whereas the target pinion angle
calculation circuit 203 of this example acquires the basic assist
component T.sub.a1*. The target steering angle calculation circuit
52 acquires the current value I.sub.b of the current supplied to
the steering operation motor 41, whereas the target pinion angle
calculation circuit 203 of this example acquires the current value
I.sub.m of the current supplied to the assist motor 191. The target
pinion angle calculation circuit 203 acquires the steering torque
T.sub.h and the vehicle speed V similarly to the target steering
angle calculation circuit 52. The target steering angle calculation
circuit 52 calculates the target steering angle .theta.*, whereas
the target pinion angle calculation circuit 203 of this example
calculates the target pinion angle .theta..sub.p*. Although a part
of the signals to be acquired and the signal to be generated are
different, details of internal calculation processing of the target
pinion angle calculation circuit 203 are the same as those of the
target steering angle calculation circuit 52.
[0174] According to this embodiment, effects similar to those of
the first embodiment can be attained. That is, the basic control
amount I.sub.1* is changed in response to the axial force deviation
.DELTA.F between the ideal axial force F.sub.i and the estimated
axial force F.sub.e, thereby calculating a basic assist component
T.sub.a1* that reflects the road condition (such as a road
frictional resistance) more appropriately. Thus, a more appropriate
assist force in response to the road condition is applied to the
steering wheel 11. The driver feels the steering reaction force via
the steering wheel 11 as tactile feedback, and can therefore grasp
the road condition more accurately.
[0175] For example, the gain G2 calculated by the gain calculation
circuit 122 (see FIG. 7) is set to a larger value as the axial
force deviation .DELTA.F increases along with a decrease in the
road grip of the tire. Therefore, the basic control amount I.sub.1*
and furthermore the basic assist component T.sub.a1* have larger
values. Thus, a more appropriate (greater) assist force in response
to the road grip of the tire is applied to the steering wheel 11.
Through a decrease in the steering torque T.sub.h along with the
increase in the assist force, the driver can grasp, as tactile
feedback, a situation in which the road grip of the tire
decreases.
[0176] The sixth embodiment may be modified as follows. In this
example, the calculation functions of the target steering reaction
force calculation circuit 51 of the second to fifth embodiments may
be provided to the basic assist component calculation circuit 202.
Also in this case, effects similar to those of the second to fifth
embodiments can be attained.
[0177] In this example, the electric power steering system (EPS)
190 configured to apply a steering assist force to the steering
operation shaft 14 is taken as an example, but another type of EPS
configured to apply a steering assist force to the steering shaft
may be employed instead. Details are as follows.
[0178] As indicated by long dashed double-short dashed lines in
FIG. 15, the assist motor 191 is coupled to the steering shaft 12
instead of the steering operation shaft 14 via the speed reducing
mechanism 192. The pinion shaft 44 may be omitted. In this case,
the control apparatus 194 executes feedback control for the
steering angle .theta..sub.s instead of the feedback control for
the pinion angle .theta..sub.p.
[0179] That is, as parenthesized in FIG. 16, the pinion angle
calculation circuit 201 functions as a steering angle calculation
circuit configured to calculate a steering angle .theta..sub.s
based on the current value I.sub.m of the assist motor 191. The
target pinion angle calculation circuit 203 functions as a target
steering angle calculation circuit configured to calculate a target
steering angle that is a target value of the steering angle
.theta..sub.s based on the steering torque T.sub.h, the vehicle
speed V, the basic assist component T.sub.a1*, and the current
value I.sub.m. The target steering angle calculation circuit
basically has a configuration similar to that of the target
steering angle calculation circuit 52 illustrated in FIG. 4. The
differentiator 79 provided in the control apparatus 194 calculates
a steering speed .omega..sub.s by differentiating the steering
angle .theta..sub.s. The pinion angle feedback control circuit 204
functions as a steering angle feedback control circuit configured
to determine a deviation between the target steering angle and the
actual steering angle .theta..sub.s, and calculate a correction
component T.sub.a2* for the basic assist component T.sub.a1* so as
to eliminate the deviation.
[0180] The embodiments may be modified as follows. In the first to
sixth embodiments, the torque sensor 34 is provided on the steering
shaft 12, but may be provided on the pinion shaft 13. The position
where the torque sensor 34 is provided is not limited as long as
the steering torque T.sub.h can be detected.
[0181] In the first to fifth embodiments, the steer-by-wire type
steering system 10 may have a configuration in which the clutch 21
is omitted. In the first to fifth embodiments, the control
apparatus 50 may have a configuration in which the differentiation
steering control circuit 63 is omitted. In this case, the pinion
angle feedback control circuit 64 acquires the target pinion angle
.theta..sub.p* calculated by the steering angle ratio change
control circuit 62, and executes feedback control for the pinion
angle .theta..sub.p so that the actual pinion angle .theta..sub.p
follows the acquired target pinion angle .theta..sub.p*.
[0182] In the first to fifth embodiments, the control apparatus 50
may have a configuration in which both the differentiation steering
control circuit 63 and the steering angle ratio change control
circuit 62 are omitted. In this case, the target steering angle
.theta.* calculated by the target steering angle calculation
circuit 52 is directly used as the target pinion angle
(.theta..sub.p*). That is, the steered wheels 16 and 16 are turned
by an amount corresponding to the operation of the steering wheel
11.
[0183] In the first to sixth embodiments, the basic control circuit
81 calculates the basic control amount I.sub.1* by multiplying the
steering torque T.sub.h and the gain G1 together, but may be
configured as follows. That is, the basic control circuit 81
calculates the basic control amount I.sub.1* by using a
three-dimensional map that defines a relationship between the
steering torque T.sub.h and the basic control amount depending on
the vehicle speed V. The basic control circuit 81 sets the absolute
value of the basic control amount I.sub.1* to a larger value as the
absolute value of the steering torque T.sub.h increases and as the
vehicle speed V decreases.
[0184] In the first to sixth embodiments the vehicle model 72 may
have a configuration in which at least one of the two estimated
axial force calculation circuits 93 and 94 is omitted. That is, the
axial force F.sub.pre may be calculated by summing up at least the
axial force F1 (estimated axial force) estimated and calculated by
the estimated axial force calculation circuit 92 and the ideal
axial force F.sub.i at a predetermined blending ratio. The final
axial force F.sub.sp is calculated by summing up the axial force
F.sub.pre and the imaginary rack end axial force F.sub.end.
[0185] In the first to sixth embodiments, the blending ratio
D.sub.j of the estimated axial force F.sub.e calculated by the
first calculation circuit 95a may be determined by using at least
one of the blending ratios Dcc, D.sub.d, D.sub.e, D.sub.f, D.sub.g,
D.sub.h, and D.sub.i calculated by the respective blending ratio
calculation circuits (108 to 114). When one of the blending ratios
is used alone, the one blending ratio is directly used as the
blending ratio D.sub.j of the estimated axial force F.sub.e.
[0186] In the first to sixth embodiments, depending on product
specifications, the vehicle speed V need not be taken into
consideration in the maps that are used in the gain calculation
circuits 122, 133, 142, 152, 156, 158, 172, 174, and 176 configured
to calculate gains based on the axial force deviation .DELTA.F.
[0187] In the first to sixth embodiments, the gain calculation
circuits 122, 133, 142, 152,156, 158, 172, 174, and 176 calculate
gains by using the axial force deviation .DELTA.F that is a
difference between the ideal axial force F.sub.i and the estimated
axial force F.sub.e, but a difference between one of the following
axial forces (B1) to (B4) and the ideal axial force F.sub.i may be
used as the axial force deviation .DELTA.F.
[0188] (B1) The axial force F1 calculated by the estimated axial
force calculation circuit 92. The axial force F1 is based on the
current value I.sub.b of the steering operation motor 41.
[0189] (B2) The axial force F2 estimated and calculated by the
estimated axial force calculation circuit 93. The axial force F2 is
based on the lateral acceleration LA.
[0190] (B3) The axial force F3 estimated and calculated by the
estimated axial force calculation circuit 94. The axial force F3 is
based on the yaw rate YR.
[0191] (B4) The axial force F.sub.c calculated by the multiplier
103 of the axial force blending calculation circuit 95. The axial
force F.sub.c is obtained by summing up the axial forces F2 and F3
at predetermined blending ratios.
[0192] In this case, the subtractor 107 illustrated in FIG. 6 may
acquire the axial force F1, the axial force F2, the axial force F3,
or the axial force F.sub.c in place of the estimated axial force
F.sub.e. As indicated by long dashed double-short dashed lines in
FIG. 6, a subtractor 107a may be added to the axial force blending
calculation circuit 95, and the added subtractor 107a may calculate
a difference between the ideal axial force F.sub.i and the axial
force F1, the axial force F2, the axial force F3, or the axial
force F.sub.c. FIG. 6 illustrates an example in which the added
subtractor 107a acquires the axial force F1.
* * * * *